Free-space optical communication links are increasingly being used for relatively short links (e.g., about 1 km), which are often established for temporary purposes (e.g., sporting event, outdoor concert venue, and the like). By their nature, these links provide the advantages of high capacity and low latency. Free-space links are similarly being explored for extremely long links (e.g., out of the earth's orbit, between satellites, etc.). In either case of “relatively short” or “extremely long” links, the sensitivity of the link is fundamentally limited by the effects of diffraction (i.e., the continual divergence of a free-space optical beam as it travels from a transmitter to a receiver). In this context, sensitivity is defined as the signal strength required to recover the transmitted information with sufficient fidelity. As there are practical limits on the size of the apertures permitted at both the transmitter and receiver, the effects of diffraction result in only a portion of the diverging beam being captured at the receiver (as a function of the aperture size/optical components at the receiver).
While improving the receiving optics allows for a larger portion of the signal to be captured, the expense associated with this approach is not always suitable for a given application. Another approach to improving the sensitivity of a free space optical receiver is to utilize advanced modulation schemes at the transmitter which allow for the data to be transmitted at lower power, or the like.
It is apparent that an alternative approach to providing a free space optic link that remains within the power budget allotted for practical applications is required.